1. Field of the Invention
The present invention relates generally to a developing device used for developing a latent image formed on an image carrying member in an image forming apparatus such as a copying machine or a printer. Particularly, it relates to a developing device so adapted as to press a controlling member against the surface of a developer carrying member while conveying a developer to a developing area opposite to an image carrying member with the developer held on the surface of the developer carrying member and control the amount of the developer conveyed to the developing area by the controlling member.
2. Description of the Related Art
In an image forming apparatus such as a copying machine or a printer, various developing devices have been conventionally used for developing a latent image formed on an image carrying member.
As such a developing device, a developing device using a monocomponent developer containing no carrier particles has been known in addition to a developing device using a two-component developer containing carrier particles and toner particles.
In such a developing device using a monocomponent developer, in order that a developer in a suitable amount is stably conveyed to a developing area opposite to an image carrying member by a developer carrying member, one so adapted as to press a controlling member having spring elasticity or rubber elasticity against the surface of a developer carrying member while conveying a developer to a developing area opposite to an image carrying member with the developer held on the surface of the developer carrying member, control the amount of the developer conveyed to the developing area with the developer held on the surface of the developer carrying member by the controlling member, and subject the developer to triboelectric charging has been generally known, as disclosed in Japanese Patent Publication No. 16736/1988.
Examples of the developer carrying member include an elastic roller composed of a rubber material as disclosed in Japanese Patent Publication No. 19749/1987 and one in a sleeve shape composed of a metal or a resin material as disclosed in Japanese Patent Laid-Open No. 226676/1988.
When the controlling member is pressed against the surface of the developer carrying member, to control the amount of the developer conveyed to the developing area as described above, however, a heavy load is applied to the developer by contact pressure of the controlling member, whereby the developer on the surface of the developer carrying member may, in some cases, be cracked, producing fine powder. Particularly in order to form an image at high speed, when the speed at which the developer is conveyed by the developer carrying member is increased, the developer is badly cracked by pressing of the controlling member, producing a lot of fine powder.
When the developer is thus cracked, producing fine powder, the fine powder is gradually increased and is welded on the surface of the developer carrying member, so that a formed image may be made non-uniform in density, for example.
In order to prevent the developer from being cracked by pressing of the controlling member, therefore, a method of weakening contact pressure of the controlling member against the developer carrying member and reducing the load applied to the developer has been considered. When the contact pressure of the controlling member is thus weakened, there are some problems. For example, it is difficult to control the amount of the developer conveyed to the developing area opposite to the image carrying member by the developer carrying member to be a suitable amount, and it is impossible to sufficiently subject the developer to triboelectric changing.